The present invention relates to a printed material imparted with improved water-proofness and a method for the preparation thereof. More particularly, the invention relates to a printed material having excellent water-proofness in which printing is made by the so-called ink-jet printing method using an aqueous ink composition which otherwise has only very poor water-proofness as well as a method for imparting excellent water-proofness to such an ink-jet printed material.
The above mentioned ink-jet printing method is one of the widely utilized printing methods for recording of characters and graphics on recording paper or sheet in output terminals of computers, word processors, facsimile machines and the like. As a principle of the ink-jet printing method, a low-viscosity ink is ejected from a number of fine nozzles each having a inner diameter of several tens of micrometers (.mu.m) under controlled ejecting pressure and accelerating voltage to form ink droplets having a diameter of 100 to 200 .mu.m with good stability which impinge at and are deposited on the recording paper or sheet as dots to form a patterned image thereon while occurrence of such ink dots is controlled in accordance with the pattern to be reproduced on the recording paper or sheet.
The above mentioned ink-jet printing method has several advantages over printing methods by other principles. For example, the printing can be performed producing no or little noise even in high-speed printing and visible printed images can be obtained directly without treatments of development and fixing. Further, plain paper can be used as the recording paper in the ink-jet printing method. The method is applicable not only to monochromatic printing but also to multi-color printing so that intensive efforts are being made for the development of an ink-jet printer capable of producing a colored hard copy from a color display and usable as the printers for facsimile machines, word processors, computer terminals and the like.
Needless to say, the ink-jet printing method is not without any problems and disadvantages. One of the major problems arises from the use of a low-viscosity aqueous ink. Namely, the ink droplets impinging at the surface of the recording sheet must infiltrate and be absorbed by the recording sheet as rapidly as possible so that at least the surface layer of the recording sheet is required to be highly water-absorptive in respect of the velocity and capacity of water absorption in order to ensure good quality of the printed images on the recording sheet. In this regard, it is widely practiced, especially when the base of the recording sheet is a film of a plastic resin having no water-absorptivity, that the surface of the recording sheet is coated with a highly water-absorptive particulate material which may be a fine powder of a highly water-absorptive resinous polymer such as a saponification products of polymers of acrylic esters, copolymers of an acrylic ester and vinyl acetate, copolymers of vinyl acetate and maleic acid and the like, an inorganic highly-porous fine powder such as synthetic silica fillers or a mixture thereof uniformly dispersed in an adhesive binder such as polyester resins, acrylic resins, polyurethane resins, epoxy resins and the like. Since the coloring material or dye contained in the aqueous ink and deposited on and absorbed by the above mentioned coating layer of the water-absorptive particulate material is soluble in water, drawbacks of spreading and blur may sometimes be caused in the printed images on such a recording sheet to decrease or lose the legibility of the recorded images when the printed material is brought into contact with water or prolongedly kept under adverse conditions of a high temperature of, for example, 40.degree. C. or higher and a high relative humidity of, for example, 80% or higher. Therefore, printed materials by the ink-jet printing method should be kept with good care not to cause such troubles. This problem is a factor greatly limiting the applicability and versatility of the ink-jet printing method even to destroy the above mentioned advantages inherent in the method.